1. Field of the Invention
The present invention relates to a wireless communication apparatus that performs data communications with an access point and to a method for controlling the wireless communication apparatus and a program therefor.
2. Description of the Related Art
Generally, a wireless local area network (LAN) system includes an access point (hereinafter referred to as AP in IEEE 802.11 and QAP in IEEE 802.11e) and a number of wireless terminals (hereinafter referred to as STAs in IEEE 802.11 and QSTAs in IEEE 802.11e). A LAN may be coupled between the access point and each of the wireless terminals by using a wireless medium (e.g., radio waves). An access point functions as a bridge or router and also communicates with a wired LAN. A wireless terminal can communicate with another wireless terminal and wired terminals participating in a wired LAN via an access point.
IEEE 802.11, a standard for a wireless LAN, defines two access control methods for providing and/or controlling access to a wireless medium between APs and STAs in the function of the Medium Access Control (MAC) layer. One is the Distributed Coordination Function (DCF), which is of a distributed control type, and the other is the Point Coordination Function (PCF), which is of a centralized control type.
Distributed-control-type DCF performs access control by using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA has an advantage that APs and STAs can transmit a packet in an autonomous and decentralized manner because a wireless terminal that tries to carry out packet transmission starts transmitting after determining that a wireless medium is not used. However, CSMA/CA has a disadvantage that an increase in the number of STAs and in the amount of traffic increases the number of collisions between packets and the number of retransmissions and thereby decreases throughput.
Centralized-control-type PCF performs access control in which an AP collectively controls access from itself and each STA to a wireless medium. Because only an STA that is polled by the AP is permitted to transmit, this lowers the degree of flexibility in transmission timing. However, collisions may not occur between packets and thus retransmission may not be necessary. Therefore, PCF may be more suitable for data transmission that requires real-time processing.
An AP can use DCF access control and PCF access control alternately and periodically. When the AP uses PCF, the AP must indicate an access control period for use in the PCF to each STA by using a management frame and control frame (e.g., beacon frame and Contention-Free End (CF-End) frame). This period is called the Contention Free Period (CFP), and during the CFP, each STA does not transmit a packet until the STA is polled by the AP.
In the IEEE 802.11 standard mentioned above, a variety of parameters in the DCF and PCF and the essential operations to be performed by the AP and STA are defined, but details of control and applications are dependent on implementations, so specifics are not defined. On the basis of this background, several techniques concerning these two types of access control have been proposed.
One such technique is disclosed in Japanese Patent Laid-Open No. 2004-158965. In a wireless LAN system that has a PCF described in this patent document, an STA sends, to an AP, a subsequent request bandwidth corresponding to the transfer amount of transmission data that is to be subsequently requested, together with the current transmission data. This technique aims to efficiently perform data transmission by dynamically controlling a bandwidth for each data transmission.
As a recent standard trend, Task Group E (TGe) is discussing a standard that includes a mechanism performing a prioritization and a bandwidth reservation depending on the contents or use of data to support the Quality of Service (QoS) and is defining such a standard as IEEE 802.11e.
In IEEE 802.11e, there are two methods of access control: the compulsory Enhanced Distributed Channel Access (EDCA) and the optional HCF, Hybrid Coordination Function, Controlled Channel Access (HCCA).
EDCA is of a distributed-control-type method, like the conventional DCF, but is enhanced so as to be able to perform prioritization with respect to access to a wireless medium depending on the type of data. In EDCA, audio data or other data, which requires real-time processing, can preferentially access a wireless medium.
HCCA is of a centralized control type method, like the conventional PCF, but, in addition to the prioritization, HCCA is designed such that efficient bandwidth reservations corresponding to the transmission conditions can be realized by exchanging necessary transmission conditions in advance between a QAP and a QSTA. In the conventional PCF, a period for which an AP can poll a wireless terminal is limited to a CFP. In contrast, for the HCCA, if a QAP detects a certain idle time in the wireless medium, the QAP can obtain access at any time, transmit data, and poll another QSTA.
The conventional wireless terminals described above are designed to perform efficient data transmission by allowing necessary transmission conditions to be exchanged in advance between a wireless terminal and an AP and thus allowing an appropriate bandwidth to be reserved by the AP. However, depending on the difference in interpretative methods between the wireless terminal and the AP or the state of radio waves, a problem arises in which too much or too little bandwidth may be reserved.
Additionally, the conventional wireless terminals are designed to perform efficient data transmission by allowing necessary transmission conditions to be exchanged in advance between a wireless terminal and an AP and thus allowing appropriate prioritization to be performed by the AP. However, depending on the difference in interpretative methods between the wireless terminal and the AP or the state of accommodating other wireless terminals, another problem arises in which too high or too low a priority may be assigned.